Heterogeneous catalysis has played a critical role in many chemical processes. The impact of heterogeneously catalyzed processes on the global economy has been estimated at 20% of the world GNP, i.e., roughly $5 trillion/year. The main industrial applications of heterogeneous catalysis are petroleum refining, chemical production, and environmental protection. Petroleum refining involves the largest volume of materials processed, with the world oil refining capacity in excess of 3.6xc3x971012 kg/year.
Acid catalysis forms the basis of the most highly utilized hydrocarbon conversion processes in the petroleum industry, and constitutes an active field of research today. Although the industrial processes, such as paraffin isomerization, alkylation, catalytic cracking, and naphtha reforming, lead to different end-products, they all depend on materials with surface acidity. Environmental problems with the upstream of the refined hydrocarbon products have goaded the search for improved acid catalysts. In the production of motor-grade fuel through alkylation of isobutane with alkenes, H2SO4 or HF is used as the catalyst. These liquid mineral acids are corrosive, dangerous to handle, and difficult to dispose of. Even some industrial solid acid catalysts are environmentally harmful. For example, the bifunctional Pt-doped chlorinated alumina catalyst used in the n-butane isomerization process requires the addition of chlorinated compounds to maintain catalytic activity because it leaches corrosive HCl during use.
More significant are the problems concerning the downstream use of the hydrocarbon products, especially the deleterious emissions from the combustion of gasoline motor fuel. Addressing this was the Clean Air Act Amendments of 1990, which mandated the reformulation of motor fuel gasoline (40-50% of all petroleum products in the US). As a result, demand for particular blend components has heightened, increasing the load on existing catalytic processes. Aluminosilicate zeolites have attempted to address these environmental issues, but there is much more room for improvement, given the development of novel solid acidic materials.
Aluminosilicate zeolites are microporous, crystalline materials composed of AlO4 and SiO4 tetrahedra arranged around highly ordered channels and/or cavities. Zeolites are acidic solids, in which the surface acidity is generated by protons required for charge balance of the framework and located near the Al cations. More generally referred to as molecular sieves, these materials have structural properties desirable for solid acid catalysts, such as surface acidity, high surface areas, and uniform pore sizes. Examples of zeolites used as solid acids in petroleum refining include Pt/mordenite for C5/C6 isomerization, ZSM-5 for xylene isomerization and methanol-to-gasoline conversion, sulfided NiMo/faujasite for hydrocracking of heavy petroleum fractions, and USY for fluidized catalytic cracking. Zeolites are also used for other acid-catalyzed processes. The main difficulty in employing zeolites as acid catalysts lies in their great tendency to deactivate and their limited usefulness in reactions involving large molecules. Zeolites are restricted to particular compositions, pore sizes and pore structures, which limit their applicability.
A plethora of non-zeolitic materials with surface acidic properties have been investigated as potential solid acid catalysts. Superacidity is beneficial for acid-catalyzed hydrocarbon reactions because lower operation temperatures are required. Moreover, superacidic materials exhibit strong acidity and high activity for hydrocarbon reactions that are difficult to catalyze. Particularly interesting are the so-called xe2x80x9csuperacidsxe2x80x9d, which have acidic strengths greater than 100% H2SO4. Sulfated zirconia and tungstated zirconia are well-studied examples of xe2x80x9csuperacidicxe2x80x9d solids. Tungstated alumina is another example of a strongly acidic material.
The most challenging aspect in the isomerization of mid-distillates is to obtain high selectivity for isomerization vs. cracking at high conversion. Sulfated zirconia is active in converting hydrocarbons even at temperatures below 100xc2x0 C., but it favors cracking reactions. Iglesia et al. (1996) found that at 200xc2x0 C. with about 50% n-heptane conversion, isomerization selectivities were 85% on Pt/WO3/ZrO2, but only 35% on Pt/SO42xe2x88x92/ZrO2. Currently, zeolites and tungstated zirconia are the two most studied solid acids for the isomerization of mid-distillates due to the selectivity and stability of these catalysts. The benefits of using non-zeolitic materials include greater compositional flexibility, and therefore greater control of surface acidity, higher thermal and hydrothermal stability, and lower catalyst cost.
In certain embodiments, the catalytic compounds of the invention are represented by the generalized formula:
R1/R4/R2-R3 
wherein:
R1 is a metal or metal alloy or bimetallic system;
R2 is any metal dopant;
R3 is a metallic oxide or mixtures of any metallic oxide;
R4 is selected from WOx, MoOx, SO42xe2x88x92 or PO43xe2x88x92; and
x is a whole or fractional number between 2 and 3 inclusive.
In a particular embodiment, R1 is selected from a Group VIII noble metal or a combination of Group VIII noble metals. In another embodiment, R1 is selected from platinum, palladium, iridium, rhodium, or a combination of these. In yet another embodiment, R1 is a Ptxe2x80x94Sn, Ptxe2x80x94Pd, or Ptxe2x80x94Ga alloy or bimetallic system.
In a particular embodiment, R2 is selected from the group Al3+, Ga3+, Ce4+, Sb5+, Sc3+, Mg2+, Co2+, Fe3+, Cr3+, Y3+, Si4+, and In3+.
In another particular embodiment, R3 is selected from the group zirconium oxide, titanium oxide, tin oxide, ferric oxide, cerium oxide or mixtures thereof. In another particular embodiment, R4 is selected from SO42xe2x88x92, WOx, MoOx, PO43xe2x88x92, W20O58, WO29 and anions and mixtures thereof. In a particular embodiment, the metallic oxide is ZrO2. In a particular embodiment, x about 3.
In one embodiment, the ratio of metal dopant to metal in the oxide is less than or equal to about 0.20. In another embodiment, the ratio of metal dopant to metal in the oxide is less than or equal to about 0.05. In yet another embodiment, the ratio of metal dopant to metal in the oxide is about 0.05.
In another embodiment, the catalytic compounds of the present invention are represented by Pt/WO3/Alxe2x80x94ZrO2 
Another aspect of this invention is a method of alkane and alkyl moiety isomerizations comprising the step of contacting a catalyst with an alkane or alkyl, wherein said catalyst comprises:
R1/R4/R2-R3 
wherein:
R1 is a metal or metal alloy or bimetallic system;
R2 is any metal dopant;
R3 is a metallic oxide or mixtures of any metallic oxide;
R4 is selected from WOx, SO42xe2x88x92, MoOx, or PO43xe2x88x92; and
x is a whole or fractional number between 2 and 3 inclusive.
In a preferred embodiment, the catalysts are used for conversion of straight chain or n-alkyls. In certain embodiments, the n-alkyl is a straight chain lower alkane, or C4-C7 alkane. In certain other embodiments, the n-alkyl is n-hexane, n-octane, or n-heptane. In a particular embodiment, the n-alkyl is n-heptane.
In one embodiment, the temperature of the reaction was lower than 210xc2x0 C., lower than 170xc2x0 C., lower than 150xc2x0 C. In another embodiment, the isomerization conversions are higher than 80%. In yet another embodiment, the catalyst compounds are used in a process to produce alkane or alkyl moiety isomers with a yield of greater than 70%, greater than 80% of the reaction product. In a further embodiment, the catalyst compounds were used to produce alkanes in the form of higher octane number, multi-branched alkanes.